From time to time it is necessary to produce a still television picture for transmission purposes from a scene which originally contained moving portions. An apparatus which is capable of freezing a moving television picture consists of a field or frame store into which television information is successively entered and from which the same information is successively retrieved for onward transmission. Such a frame store is disclosed in the processing system of U.S. Pat. No. 4,148,070 and which store is the subject of U.S. patent application Ser. No. 764,148 (and British Patent Application No. 6585/76). A freeze is initiated by inhibiting the writing process so that the data contained within the store at any one moment is cyclically read and transmitted onwards.
If the store comprises a single field store, then the field is repeated to produce a two-field system of interlace according to the requirements of a standard television system, such as PAL or NTSC.
If the store comprises a framestore, it is necessary to modify the operation during a freeze as, during the writing process that preceded the freeze operation, some difference between the two fields in store due to movement may have taken place. It is normal to prevent this movement flicker by inhibiting the operation of one or other of the fields in store and making use of a repeated single field sequence rather than selecting the two field read out which could be used if the picture is completely stationary (see also FIG. 1).
A main disadvantage of such a system is the reduction in vertical resolution caused by using a single field system which effectively halves the vertical bandwidth. It is an intention of the present invention to eliminate or reduce this disadvantage in order to produce a frozen picture, which contains the full vertical detail where possible.